Suspension system for a vehicle

ABSTRACT

Suspension system for a vehicle is disclosed and claimed. The suspension system includes a compressible fluid, a suspension strut, a hydraulic cavity, a reservoir, and a volume modulator. The hydraulic cavity is at least partially defined by the suspension strut and is adapted to contain a portion of the compressible fluid. The hydraulic cavity and the compressible fluid supply a suspending spring force that biases a wheel of a vehicle toward the road surface. The volume modulator selectively pushes the compressible fluid into the hydraulic cavity and vents the compressible fluid from the hydraulic cavity, thereby actively modulating the suspending spring force.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention is a continuation-in-part of InternationalApplication No. PCT/US01/48488, filed 07 Dec. 2001 and entitled“Suspension System For A Vehicle”, which claims benefit of U.S.provisional application Ser. No. 60/251,951, filed 07 Dec. 2000 andentitled “Compressible Fluid Strut”.

TECHNICAL FIELD

The subject matter of this invention generally relates to suspensionsystems for a vehicle and, more particularly, to suspension systemsincluding a compressible fluid.

BACKGROUND

In the typical vehicle, a combination of a coil spring and a gas strutfunction to allow compression movement of a wheel toward the vehicle andrebound movement of the wheel toward the ground. The suspension strutsattempt to provide isolation of the vehicle from the roughness of theroad and resistance to the roll of the vehicle during a turn. Morespecifically, the typical coil spring provides a suspending spring forcethat biases the wheel toward the ground and the typical gas strutprovides a damping force that dampens both the suspending spring forceand any impact force imparted by the road. Inherent in everyconventional suspension strut is a compromise between ride (the abilityto isolate the vehicle from the road surface) and handling (the abilityto resist roll of the vehicle). Vehicles are typically engineered formaximum road isolation (found in the luxury market) or for maximum rollresistance (found in the sport car market). There is a need, however,for an improved suspension system that avoids this inherent compromise.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cut away perspective view of the suspension system of thepreferred embodiment, shown within a vehicle.

FIG. 2 is a schematic view of the suspension system of FIG. 1.

FIG. 3 is a cross-sectional view of a suspension strut of the suspensionsystem of FIG. 1.

FIG. 4 is a detailed view of the volume modulator of the suspensionsystem of FIG. 1.

FIGS. 5A, 5B, 6A, and 6B are schematic views of the different stages ofthe volume modulator of FIG. 4.

FIG. 7 is a schematic view of the volume modulator of a second preferredembodiment.

FIGS. 8A, 8B, 9A, and 9B are schematic views of the different stages ofthe volume modulator of FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiments of the inventionis not intended to limit the invention to these preferred embodiments,but rather to enable any person skilled in the art of suspension systemsto use this invention.

As shown in FIG. 1, the suspension system 10 of the preferred embodimentincludes a compressible fluid 12, a suspension strut 14, a hydrauliccavity 16, a reservoir 18, and a volume modulator 20. The hydrauliccavity 16, which is at least partially defined by the suspension strut14, contains a portion of the compressible fluid 12 and cooperates withthe compressible fluid 12 to supply a suspending spring force. Thesuspending spring force biases a wheel 22 of the vehicle 24 toward thesurface. The volume modulator 20, which is coupled to the hydrauliccavity 16 and to the reservoir 18, selectively pushes the compressiblefluid 12 from the reservoir 18 into the hydraulic cavity 16 and ventsthe compressible fluid 12 from the hydraulic cavity 16 into thereservoir 18, thereby actively modulating the suspending spring force.By increasing the suspending spring force in the suspension struts 14 ofthe outside wheels during a turn, the vehicle 24 can better resist roll.By decreasing the suspending spring force over rough surfaces, thevehicle 24 can better isolate the passengers. Thus, by activelymodulating the suspending spring force, the vehicle 24 can maximize bothride and handling and avoid the inherent compromise of conventionalsuspension systems.

As shown in FIGS. 1 and 2, the suspension system 10 of the preferredembodiment has been specifically designed for a vehicle 24 having fourwheels 22 and four suspension links 26 (two shown in FIG. 2) suspendingthe individual wheels 22 from the vehicle 24. The suspension links 26allow compression movement of the individual wheels 22 toward thevehicle 24 and rebound movement of the individual wheels toward the roadsurface. Despite this design for a particular environment, thesuspension system 10 may be used in any suitable environment, such asother vehicles with more or less wheels.

The compressible fluid 12 of the preferred embodiment, which cooperatesto supply the suspending spring force, is preferably a silicon fluidthat compresses about 1.5% volume at 2,000 psi, about 3% volume at 5,000psi, and about 6% volume at 10,000 psi. Above 2,000 psi, thecompressible fluid has a larger compressibility than conventionalhydraulic oil. The compressible fluid, however, may alternatively be anysuitable fluid, with or without a silicon component, that provides alarger compressibility above 2,000 psi than conventional hydraulic oil.

As shown in FIGS. 2 and 3, the suspension strut 14 of the preferredembodiment includes a hydraulic tube 28, a displacement rod 30, a cavitypiston 32, a first variable restrictor 34, and a second variablerestrictor 36. The hydraulic tube 28 and displacement rod 30 of thepreferred embodiment cooperatively function to couple the suspensionlink and the vehicle and to allow compression movement of the wheel 22toward the vehicle and rebound movement of the wheel 22 toward the roadsurface. The hydraulic tube 28 preferably defines an inner cavity 38,which functions to contain a portion of the compressible fluid 12. Aspreviously mentioned, the inner cavity 38 and the compressible fluid 12preferably cooperate to supply the suspending spring force that biasesthe wheel 22 toward the surface and, essentially, suspend the entirevehicle above the surface. The displacement rod 30 is adapted to moveinto the inner cavity 38 upon the compression movement of the wheel 22and to move out of the inner cavity 38 upon the rebound movement of thewheel 22. As it moves into the inner cavity 38, the displacement rod 30displaces, and thereby compresses, the compressible fluid 12. In thismanner, the movement of the displacement rod 30 into the inner cavity 38increases the suspending spring force of the suspension strut 14. As thedisplacement rod 30 moves out of the inner cavity 38, the compressiblefluid 12 decompresses and the suspending spring force of the suspensionstrut 14 decreases. The displacement rod 30 is preferably cylindricallyshaped and, because of this preference, the displacement of thedisplacement rod 30 within the inner cavity 38 and the magnitude of thesuspending spring force have a linear relationship. If a linearrelationship is not preferred for the particular application of thesuspension strut 14, or if there is any other appropriate reason, thedisplacement rod 30 may be alternatively designed with another suitableshape. The hydraulic tube 28 and the displacement rod 30 are preferablymade from conventional steel and with conventional methods, but mayalternatively be made from any suitable material and with any suitablemethod.

The cavity piston 32 of the preferred embodiment is preferably coupledto the displacement rod 30 and preferably extends to the hydraulic tube28. In this manner, the cavity piston 32 separates the inner cavity 38into a first section 40 and a second section 42. The cavity piston 32defines a first orifice 44 and a second orifice 46, which bothpreferably extend between the first section 40 and the second section 42of the inner cavity 38. The first orifice 44 and the second orifice 46function to allow flow of the compressible fluid 12 between the firstsection 40 and the second section 42 of the inner cavity 38. The cavitypiston 32 is preferably securely mounted to the displacement rod 30 by aconventional fastener 48, but may alternatively be integrally formedwith the displacement rod 30 or securely mounted with any suitabledevice. The cavity piston 32 is preferably made from conventionalmaterials and with conventional methods, but may alternatively be madefrom other suitable materials and with other suitable methods.

The first variable restrictor 34 of the preferred embodiment is coupledto the cavity piston 32 near the first orifice 44. The first variablerestrictor 34 functions to restrict the passage of the compressiblefluid 12 through the first orifice 44 and, more specifically, functionsto variably restrict the passage based on the velocity of the cavitypiston 32 relative to the hydraulic tube 28. In the first preferredembodiment, the first variable restrictor 34 is a first shim stack 50preferably made from conventional materials and with conventionalmethods. In alternative embodiments, the first variable restrictor 34may include any other suitable device able to variably restrict thepassage of the compressible fluid 12 through the first orifice 44 basedon the velocity of the cavity piston 32 relative to the hydraulic tube28. The second variable restrictor 36 of the preferred embodiment iscoupled to the cavity piston 32 near the second orifice 46. The secondvariable restrictor 36—like the first variable restrictor 34—functionsto restrict the passage of the compressible fluid 12 through the secondorifice 46 and, more specifically, functions to variably restrict thepassage based on the velocity of the cavity piston 32 relative to thehydraulic tube 28. In the preferred embodiment, the second variablerestrictor 36 is a second shim stack 52 preferably made fromconventional materials and with conventional methods. In alternativeembodiments, the second variable restrictor 36 may include any suitabledevice able to variably restrict a passage of the compressible fluid 12through the second orifice 46 based on the velocity of the cavity piston32 relative to the hydraulic tube 28.

The cavity piston 32, the first orifice 44, and the first variablerestrictor 34 of the preferred embodiment cooperate to supply therebound damping force during the rebound movement of the wheel 22. Therebound damping force acts to dampen the suspending spring force thattends to push the displacement rod 30 out of the hydraulic tube 28. Thecavity piston 32, the second orifice 46, and a second variablerestrictor 36, on the other hand, cooperate to supply the compressiondamping force during the compression movement of the wheel 22. Thecompression damping force acts to dampen any impact force that tends topush the displacement rod 30 into the hydraulic tube 28.

The suspension strut 14 of the preferred embodiment is further describedin U.S. application filed on 07 Dec. 2001, entitled “Compressible FluidStrut”, and assigned to Visteon Global Technologies, Inc. As describedin that application, the suspension strut may include a pressure vesseland may include a valve. In alternative embodiments, the suspensionstrut may include any suitable device to allow active modulation of thesuspending spring force with compressible fluid.

As shown in FIG. 1, the suspension system 10 of the preferred embodimentalso includes hydraulic lines 54 adapted to communicate the compressiblefluid 12 between the individual suspension struts 14 and the volumemodulator 20. Together with the inner cavity 38 of the individualsuspension struts 14, the hydraulic lines 54 define individual hydrauliccavities 16. Preferably, the compressible fluid 12 flows freely betweenthe volume modulator 20 and the inner cavity 38 of the individualsuspension struts 14. Alternatively, the hydraulic cavities 16 mayinclude one or more controllable valves such that the hydraulic cavity16 is entirely defined by the suspension strut 14 or by the suspensionstrut 14 and a portion of the hydraulic line 54.

As shown in FIG. 2, the reservoir 18 functions to contain a portion ofthe compressible fluid 12 that has been vented from the hydraulic cavity16 and that may eventually be pushed into the hydraulic cavity 16. Thereservoir 18 is preferably made from conventional materials and withconventional methods, but may alternatively be made from any suitablematerial and with any suitable method. The suspension system 10 of thepreferred embodiment includes a pump 56 adapted to pressurize thecompressible fluid 12 within the reservoir 18. In this manner, thereservoir 18 acts as an accumulator. By using compressible fluid 12under a pressure of about 1500 psi within the reservoir 18, the volumemodulator 20 consumes less energy to reach a particular pressure withinan individual hydraulic cavity 16. In an alternative embodiment, thecompressible fluid 12 within the reservoir 18 may be at atmosphericpressure or may be vented to the atmosphere.

As shown in FIG. 2, the volume modulator 20 is coupled to the hydraulicline 54 and to the reservoir 18. The volume modulator 20, as previouslymentioned, functions to selectively push the compressible fluid 12 intothe hydraulic cavity 16 and to vent the compressible fluid 12 from thehydraulic cavity 16. In the preferred embodiment, the volume modulator20 is a digital displacement pump/motor as described in U.S. Pat. No.5,259,738 entitled “Fluid-Working Machine” and issued to Salter et al.on 09 Nov. 1993, which is incorporated in its entirety by thisreference. In alternative embodiments, the volume modulator 20 may beany suitable device that selectively pushes the compressible fluid 12into the hydraulic cavity 16 and vents the compressible fluid 12 fromthe hydraulic cavity 16 at a sufficient rate to actively modulate thesuspending spring force.

As shown in FIG. 4, the volume modulator 20 of the preferred embodimentdefines a modulator cavity 60 and includes a modulator piston 62 adaptedto continuously cycle through a compression stroke and an expansionstroke within the modulator cavity 60. The modulator piston 62 ispreferably connected to an eccentric 64 that is rotated by a motor 66(shown in FIG. 1). Because of the “active” nature of the modulation ofthe suspending spring force, the modulator piston 62 cycles through thecompression stroke and expansion stroke at a relatively high frequency(up to 30 Hz) and, thus, the motor preferably rotates at a relativelyhigh rotational velocity (up to 2000 rpm).

The volume modulator 20 of the preferred embodiment also includes avalve system 67, which includes a cavity-side valve 68 coupled betweenthe hydraulic line and the volume modulator 20 and a reservoir-sidevalve 70 coupled between the reservoir and the volume modulator 20. Thecavity-side valve 68 and the reservoir-side valve 70 function toselectively restrict the passage of the compressible fluid. Preferably,the cavity-side valve 68 and the reservoir-side valve 70 are so-calledpoppet valves that may be actuated at relatively high frequencies.Alternatively, the cavity-side valve 68 and the reservoir-side valve 70may be any suitable device that selectively restricts the passage of thecompressible fluid at an adequate frequency.

As shown in FIGS. 5A and 5B, the cavity-side valve 68, thereservoir-side valve 70, and the modulator piston 62 can cooperate todraw compressible fluid 12 from the reservoir and push the compressiblefluid 12 into the hydraulic cavity. In the first stage, as shown in FIG.5A, the cavity-side valve 68 is closed and the reservoir-side valve 70is opened, while the modulator piston 62 increases the volume in themodulator cavity 60 (the expansion stroke). The expansion stroke of themodulator piston 62 draws the compressible fluid 12 into the modulatorcavity 60. During the second stage, as shown in FIG. 5B, thereservoir-side valve 70 is closed and the cavity-side valve 68 isopened, while the modulator piston 62 decreases the volume in themodulator cavity 60 (the compression stroke). The compression stroke ofthe modulator piston 62 pushes the compressible fluid 12 into thehydraulic cavity, which increases the suspending spring force at thatparticular suspension strut and wheel.

As shown in FIGS. 6A and 6B, the cavity-side valve 68, thereservoir-side valve 70, and the modulator piston 62 can also cooperateto draw compressible fluid 12 from the hydraulic cavity and vent thecompressible fluid 12 into the reservoir. In the first stage, as shownin FIG. 6A, the cavity-side valve 68 is opened and the reservoir-sidevalve 70 is closed, while the modulator piston 62 increases the volumein the modulator cavity 60 and draws the compressible fluid 12 into themodulator cavity 60. During the second stage, as shown in FIG. 6B, thereservoir-side valve 70 is opened and the cavity-side valve 68 isclosed, while the modulator piston 62 decreases the volume in themodulator cavity 60 and vents the compressible fluid 12 into thereservoir, which decreases the suspending spring force at thatparticular suspension strut and wheel.

During the operation of the vehicle, it may be advantageous to neitherincrease nor decrease the suspending spring force. Since the motor 66,the eccentric 64, and the modulator pistons 62 are continuously moving,the reservoir-side valve 70 and the volume modulator 20 can alsocooperate to draw compressible fluid 12 from the reservoir (shown inFIG. 5A) and vent the compressible fluid 12 back into the reservoir(shown in FIG. 6B). This process does not modulate the pressure of thehydraulic cavity 16 and does not increase or decrease the suspendingspring force.

Although FIGS. 5A, 5B, 6A, and 6B show only one modulator cavity 60 andmodulator piston 62, the volume modulator 20 preferably includes amodulator cavity 60, a modulator piston 62, a cavity-side valve 68, anda reservoir-side valve 70 for each suspension strut 14 on the vehicle24. Preferably, the motor 66 and the eccentric 64 drive the multiplemodulator pistons 62, but the individual modulator pistons 62 mayalternatively be driven by individual motors and individual eccentrics.Further, a control unit 72 (shown in FIG. 1) may individually controlthe cavity-side valve 68 and reservoir-side valve 70 corresponding to aparticular suspension strut 14 and wheel 22 to adjust the ride andhandling of the vehicle 24 on a wheel-to-wheel basis. The control unit72 may also be used to adjust particular suspension struts 14 on aside-by-side basis of the vehicle 24 to adjust the roll or the pitch ofthe vehicle 24. The control unit 72 may further be used to adjust all ofthe suspension struts 14 to adjust the ride height of the vehicle 24.The control unit 72 is preferably made from conventional material andwith conventional methods, but may alternatively be made from anysuitable material and with any suitable method.

As shown in FIG. 7, the volume modulator 120 of the second preferredembodiment shares many components with the volume modulator 20 of thefirst preferred embodiment, including a modulator cavity 60, a modulatorpiston 62, an eccentric 64, a motor 66 (shown in FIG. 1), and a valvesystem 167. Like the valve system 67 of the first preferred embodiment,the valve system 167 of the second preferred embodiment functions toselectively restrict the passage of the compressible fluid between thehydraulic cavity and the modulator cavity 60 or restrict the passage ofthe compressible fluid between the reservoir and the modulator cavity60. The valve system 167 of the second preferred embodiment, however,includes a rotary valve 174 coupled between the modulator cavity 60, thehydraulic cavity, and the reservoir.

As shown in FIG. 8A, the rotary valve 174 of the second preferredembodiment is adapted to selectively rotate to a first position therebyrestricting the passage of the compressible fluid between the hydrauliccavity and the modulator cavity 60 and allowing the passage of thecompressible fluid between the reservoir and the modulator cavity 60. Asshown in FIG. 8B, the rotary valve 174 is further adapted to selectivelyrotate to a second position thereby restricting the passage of thecompressible fluid between the reservoir and the modulator cavity 60 andallowing the passage of the compressible fluid between the hydrauliccavity and the modulator cavity 60.

As shown in FIGS. 8A and 8B, the rotary valve 174 and the modulatorpiston 62 can cooperate to draw compressible fluid 12 from the reservoirand push the compressible fluid 12 into the hydraulic cavity. In thefirst stage, as shown in FIG. 8A, the rotary valve 174 is rotated intothe first position, while the modulator piston 62 increases the volumein the modulator cavity 60 (the expansion stroke). The expansion strokeof the modulator piston 62 draws the compressible fluid 12 into themodulator cavity 60. During the second stage, as shown in FIG. 8B, therotary valve 174 is rotated into the second position, while themodulator piston 62 decreases the volume in the modulator cavity 60 (thecompression stroke). The compression stroke of the modulator piston 62pushes the compressible fluid 12 into the hydraulic cavity, whichincreases the suspending spring force at that particular suspensionstrut and wheel.

As shown in FIGS. 9A and 9B, the rotary valve 174 and the modulatorpiston 62 can also cooperate to draw compressible fluid 12 from thehydraulic cavity and vent the compressible fluid 12 into the reservoir.In the first stage, as shown in FIG. 9A, the rotary valve 174 is rotatedinto the second position, while the modulator piston 62 increases thevolume in the modulator cavity 60 and draws the compressible fluid 12into the modulator cavity 60. During the second stage, as shown in FIG.9B, the rotary valve 174 is rotated into the first position, while themodulator piston 62 decreases the volume in the modulator cavity 60 andvents the compressible fluid 12 into the reservoir, which decreases thesuspending spring force at that particular suspension strut and wheel.

As mentioned above, during the operation of the vehicle, it may beadvantageous to neither increase nor decrease the suspending springforce. Since the motor 66, the eccentric 64, and the modulator pistons62 are continuously moving, the rotary valve 174 and the volumemodulator 20 can also cooperate to draw compressible fluid 12 from thereservoir (shown in FIG. 8A) and vent the compressible fluid 12 backinto the reservoir (shown in FIG. 9B). This process does not modulatethe pressure of the hydraulic cavity and does not increase or decreasethe suspending spring force.

As shown in FIG. 7, the valve system 167 of the second preferredembodiment also includes a hydraulic cavity valve 176 coupled along thehydraulic cavity between the volume modulator 120 and the suspensionstrut. The hydraulic cavity valve 176 functions to selectively restrictthe passage of the compressible fluid through the hydraulic cavityduring an “off” condition of the suspension system, while selectivelyallowing the passage of the compressible fluid through the hydrauliccavity during an “on” condition of the suspension system, as shown inFIGS. 8A, 8B, 9A, and 9B. Preferably, the hydraulic cavity valve 176 isa so-called poppet valve with sufficient sealing properties.Alternatively, the hydraulic cavity valve 176 may be any suitable devicethat selectively restricts the passage of the compressible fluid duringthe “off” condition of the suspension system. When using the valvesystem 167 with the hydraulic cavity valve 176, the sealing requirementsof the rotary valve 174 are reduced.

Although FIGS. 8A, 8B, 9A, and 9B show only one modulator cavity 60 andmodulator piston 62, the volume-modulator 120 preferably includes atleast one modulator cavity 60, a modulator piston 62, and a valve system167 with a rotary valve 174 for each suspension strut 14 on the vehicle24. The control unit 72 (shown in FIG. 1) may individually control therotary valve 174 corresponding to a particular suspension strut 14 andwheel 22 to adjust the ride and handling of the vehicle 24 on awheel-to-wheel basis. The control unit 72 may also be used to adjustparticular suspension struts 14 on a side-by-side basis of the vehicle24 to adjust the roll or the pitch of the vehicle 24. The control unit72 may further be used to adjust all of the suspension struts 14 toadjust the ride height of the vehicle 24.

As any person skilled in the art of suspension systems will recognizefrom the previous detailed description and from the figures and claims,modifications and changes can be made to the preferred embodiment of theinvention without departing from the scope of this invention defined inthe following claims.

1. A suspension system for a vehicle having a wheel contacting a surfaceunder the vehicle and a suspension link suspending the wheel from thevehicle and allowing relative movement of the wheel and the vehicle,said suspension system comprising: a compressible fluid; a suspensionstrut adapted to couple the suspension link and the vehicle; a hydrauliccavity at least partially defined by said suspension strut and adaptedto contain a portion of said compressible fluid and to cooperate withsaid compressible fluid to supply a suspending spring force that biasesthe wheel toward the surface; a reservoir adapted to contain a portionof said compressible fluid; and a volume modulator coupled to saidhydraulic cavity and said reservoir and adapted to selectively push saidcompressible fluid into said hydraulic cavity and vent said compressiblefluid from said hydraulic cavity, thereby actively modulating saidsuspending spring force, wherein said volume modulator defines amodulator cavity and includes a modulator piston adapted to cyclethrough a compression stroke and an expansion stroke within saidmodulator cavity, and includes a valve system adapted to selectivelyrestrict the passage of said compressible fluid between said hydrauliccavity and said modulator cavity or restrict the passage of saidcompressible fluid between said reservoir and said modulator cavity,said valve system including a rotary valve coupled between saidmodulator cavity, said hydraulic cavity, and said reservoir, said rotaryvalve being adapted to selectively rotate to a first position therebyrestricting the passage of said compressible fluid between saidhydraulic cavity and said modulator cavity and allowing the passage ofsaid compressible fluid between said reservoir and said modulatorcavity.
 2. The suspension system of claim 1 wherein said suspensionstrut includes a displacement rod adapted to move into said hydrauliccavity and to compress said compressible fluid upon the relativemovement of the wheel and the vehicle.
 3. The suspension system of claim2 wherein said displacement rod includes a cavity piston adapted tosupply a damping force.
 4. The suspension system of claim 1 wherein saidhydraulic cavity is defined by said suspension strut and a hydraulicline adapted to communicate said compressible fluid between saidsuspension strut and said volume modulator.
 5. The suspension system ofclaim 1 wherein said compressible fluid includes a silicone fluid. 6.The suspension system of claim 1 wherein said compressible fluid has alarger compressibility above 2,000 psi than hydraulic oil.
 7. Thesuspension system of claim 1 wherein said compressible fluid is adaptedto compress about 1.5% volume at 2,000 psi, about 3% volume at 5,000psi, and about 6% volume at 10,000 psi.
 8. The suspension system ofclaim 1, wherein said rotary valve is further adapted to selectivelyrotate to a second position thereby restricting the passage of saidcompressible fluid between said reservoir and said modulator cavity andallowing the passage of said compressible fluid between said hydrauliccavity and said modulator cavity.
 9. The suspension system of claim 8further comprising an electric control unit coupled to said rotary valveand adapted to rotate said valve into the first position during saidexpansion stroke and to rotate said rotary valve into the secondposition during said compression stroke, thereby pushing saidcompressible fluid into said hydraulic cavity.
 10. The suspension systemof claim 9 wherein said electric control unit is further adapted torotate said valve into the second position during said expansion strokeand to rotate said rotary valve into the first position during saidcompression stroke, thereby venting said compressible fluid from saidhydraulic cavity.
 11. The suspension system of claim 1 furthercomprising a hydraulic cavity valve coupled along said hydraulic cavitybetween said volume modulator and said suspension strut and adapted toselectively restrict the passage of said compressible fluid throughsaid-hydraulic cavity end to selectively allow the passage of saidcompressible fluid through said hydraulic cavity.